Guidelines for Characterizing "OAM"

Historically, the terms "in-band" and "out-of-band" were used extensively in radio communications as well as in telephony signaling . In both these cases, there is an actual "Band" (i.e., a "Channel" or "Frequency") to be within or outside. While those terms, useful in their simplicity, continued to be broadly used to mean "within something" and "outside something", a challenge is presented for IP communications and packet-switched networks (PSNs) which do not have a "band" per se, and, in fact, have multiple "somethings" that OAM traffic can be carried within or outside. A frequently encountered case is the use of "in-band" to mean either In-Packet or on-path. Within the IETF, the terms "in-band" and "out-of-band" cannot be reliably understood consistently and unambiguously. Context-specific definitions of these terms are inconsistent and therefore cannot be generalized. More importantly, the terms are not self-defining to any further extent and cannot be understood by someone exposed to them for the first time, since there is no "band" in IP. There are many examples of "in-band OAM" and "out-of-band OAM" in published RFCs. For instance, the term "in-band" appears in both Virtual Circuit Connectivity Verification (VCCV) and OAM for Deterministic Networking (DetNet) . While the context in each of these documents is clear, the term carries different meanings in each case. These two examples, as well as other examples of uses of the term "in-band" in previous documents are described throughout . While interpreting existing documents, it is important to understand the semantics of what "band" is a proxy for, and to be more explicit if those documents are updated. This document does not change the meaning of any terms in any prior RFCs.

This document recommends avoiding the terms "in-band" and "out-of-band" when referring to OAM. Instead, it encourages the use of more fine-grained and descriptive terminology. The document also presents alternative terms and definitions for use in future IETF documents referencing OAM, without precluding the use of other precise, descriptive terms that do not rely on the "-band" convention. The terminology presented in this section classifies OAM according to three criteria: whether it operates in an active, passive, or hybrid mode; whether it follows the same path as data traffic; and whether it receives the same treatment as data traffic.

provides clear definitions for active and passive performance assessment, enabling the construction of metrics and methods to be described as either "Active" or "Passive". Even though does not explicitly use these terms as modifiers of "OAM", they are widely used in practice and are included here for clarity. The terms "Active", "Passive" and "Hybrid", as described below, are consistent with .
Depends on dedicated OAM packets.
Depends solely on the observation of one or more existing data packet streams and does not use dedicated OAM packets.
Uses augmentation or modification of the packet stream. Examples of protocols classified as "Hybrid OAM" include Alternate Marking , In situ OAM (IOAM) , and MPLS Loss Measurement . Hybrid OAM can be implemented by piggybacking OAM-related information onto data packets, as described in , or by utilizing reserved fields in the packet header or specific values of existing header fields, as proposed in . Direct loss measurment is an example of "Hybrid OAM" in which user packets are not modified by the protocol. Instead, OAM packets are used to exchange information about user packet counters, allowing for packet loss computation. This document defines the term In-Packet OAM as a more specific and narrowly scoped instance within the broader category of Hybrid OAM.
The OAM information is carried in the packets that also carry the data traffic. This is a specific case of Hybrid OAM. It was sometimes referred to as "in-band". The MPLS echo request/reply messages are an example of "Active OAM", since they are described as "An MPLS echo request/reply is a (possibly MPLS-labeled) IPv4 or IPv6 UDP packet". IOAM is an example of "Hybrid OAM" that is also "In-Packet OAM", given that it: '...records OAM information within the packet while the packet traverses a particular network domain. The term "in situ" refers to the fact that the OAM data is added to the data packets rather than being sent within packets specifically dedicated to OAM.' Another example of In-Packet OAM is Alternate Marking , in which a small number of bits in the packet header is used for marking a subset of packets in a flow. An example of "Hybrid OAM" which is not classified as "In-Packet OAM" is Direct loss measurement . Initially, "In situ OAM" was also referred to as "In-band OAM", but was renamed due to the overloaded meaning of "In-band OAM". Further, also intertwines the terms "in-band" with "in situ", though settled on using "in Situ". Other similar uses, including and , still use variations of "in-band", "in band", or "inband".

The OAM information follows the exact same path as the observed data traffic. This was sometimes referred to as "in-band".
The OAM information does not follow the exact same path as the observed data traffic. This can also be called Path-Incongruent OAM, and was sometimes referred to as "out-of-band". In this document, the term "path-congruent packets" describes packets that follow the exact same path (i.e., traverse the same nodes and links) within a network. Note that this definition does not describe how the packets are treated in queues within the nodes on the path. A further concept, "equal-forwarding-treatment" describes how path-congruent packets receive the same forwarding treatment (e.g., Quality of Service (QoS)). An example of "Path-Congruent OAM" is the Virtual Circuit Connectivity Verification (VCCV), described is as "The VCCV message travels in-band with the Session and follows the exact same path as the user data for the session". Thus, the term "in-band" in refers to using the same path as the user data. This term is also used in Section 2 of with the same meaning, and the word "congruent" is mentioned as synonymous.

The OAM packets receive the same forwarding (e.g., QoS) treatment as user data packets. This was sometimes referred to as "in-band".
The OAM packets receive different forwarding (e.g., QoS) treatment as user data packets. This was sometimes referred to as "out-of-band". The motivation for Equal-Forwarding-Treatment OAM lies in the desire to ensure that OAM packets experience the same network conditions as the user data they are intended to monitor. This includes not only traversing the same topological path but also receiving identical Quality of Service (QoS) treatment, such as queuing, scheduling, and traffic shaping. When both topological and forwarding treatment equivalence are achieved, the OAM packets are said to exhibit fate-sharing with the data traffic. Fate-sharing ensures that any impairments or anomalies affecting the user traffic are also reflected in the behavior of the OAM packets, thereby making the results of the OAM observations more operationally meaningful and actionable. Without such equivalence, discrepancies in treatment could lead to misleading measurements or diagnostics, and even inadequate corrective actions, reducing the utility of the OAM mechanism for performance monitoring and fault detection. An example of "Equal-Forwarding-Treatment OAM" is presented in in the context of DetNet OAM: "it traverses the same set of links and interfaces receiving the same QoS and Packet Replication, Elimination, and Ordering Functions (PREOF) treatment as the monitored DetNet flow". This is classified in as "In-band OAM". Similarly, the property of "Different-Forwarding-Treatment OAM" can be found in the following definition in : "Out-of-band OAM: an active OAM method whose path through the DetNet domain may not be topologically identical to the path of the monitored DetNet flow, its test packets may receive different QoS and/or PREOF treatment, or both." uses similar text.

OAM protocols and tools can be classified according to the three criteria that were described in the previous sections. However, not all criteria are applicable to all OAM protocols, and not all combinations are necessarily possible. When defining a new OAM protocol or analyzing an existing one, it is recommended to explicitly consider which of these criteria are applicable and to describe the protocol accordingly. As a first step, all OAM mechanisms can be classified according to the first criterion, as Active, Passive, or Hybrid/In-Packet. Further classification according to the other two criteria should be considered on a case-by-case basis. In some cases, certain criteria are not relevant, or not all combinations are possible. For example: Passive OAM relies solely on observing existing data traffic and does not generate dedicated OAM packets. As such, the path congruence and forwarding treatment criteria are not relevant, since no dedicated OAM packets are exchanged between the measurment points. Non-Path-Congruent OAM, by nature, cannot be Equal-Forwarding-Treatment. A few example of OAM classification according to the three criteria are presented below: IP Ping, which uses ICMP Echo messages, can be classified as Active OAM. Since it is not guaranteed to follow the same path or receive the same treatment as user data packets, it is classified as Non-Path-Congruent and, consequently, as Different-Forwarding-Treatment. When IOAM is incorporated in data packets it can be classified as In-Packet, Path-Congruent and Equal-Forwarding-Treatment. VCCV , as discussed above, is classified as Active, Path-Congruent and Different-Forwarding-Treatment. MPLS inferred loss measurement uses specially generated test messages, and therefore can be classified as Active. It is also Path-Congruent, and can be deployed either as Equal- or Different-Forwarding-Treatment OAM. MPLS direct loss measurement uses OAM messages that exchange counters that count user data traffic. Hence, it is classified as Hybrid OAM, and as in the inferred mode, it is Path-Congruent, and can be either Equal- or Different-Forwarding-Treatment OAM. This multi-dimensional classification enables a more precise and consistent understanding of OAM mechanisms.